Oxidic crystals and v2o5-containing flux growth thereof

ABSTRACT

D R A W I N G GROWTH OF CRYSTALS OF OXIDIC MATERIALS OF THE GARNET, ORTHOFERRITE AND SPINEL FERRITE STRUCTURES FROM LEAD OXIDECONTAINING FLUXES TO WHICH V2O5 IS ADDED YIELDS CRYSTALS OF INCREASED SIZE. IN CERTAIN SYSTEMS, NOTABLY OF THE ORTHOFERRITE STRUCTURE, CONTROL OF EASY DIRECTION OF MAGNETIZATION IS AFFORDED SO THAT PLATES WITH THE EASY DIRECTION NORMAL TO THE PLANE MAY BE REPRODUCIBLY GROWN.

Sept. 5, 1972 J- P. REMEIKA OXIDIC CRYSTALS AND ho -CONTAINING FLUX GROWTH THEREOF Filed May 18, 1970 -76.1 "C"OR EASY DIRECTION OF MAGNETISATION REGISTER I, I3 I3 REGISTEROOO & G i

| i9 :3 c: 7| ;:2 I3 I I I3 l3 :3 I I L J 12 REGISTERSOO 1F REGISTERSOI TRANSFER IN PLANE CIRCUIT FIELD l4 SOURCE INPUT- CONTROL |s OUTPUT CIRCUIT cmcun INVENTOR JR REME/KA ATTORNEY-J United States Patent Office Patented Sept. 5, 1972 US. Cl. 252-62.61 12 Claims ABSTRACT OF THE DISCLOSURE Growth of crystals of oxidic materials of the garnet, orthoferrite and spinel ferrite structures from lead oxidecontaining fluxes to which V is added yields crystals of increased size. In certain systems, notably of the orthoferrite structure, control of easy direction of magnetization is aflorded so that plates with the easy direction normal to the plane may be reproducibly grown.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention is concerned with the flux growth of oxidic materials of the garnet, orthoferrite, and spinel ferrite structures, to crystals so produced and to devices using such materials. From the technological standpoint, it is significant that the class includes ferrimagnetic and canted spin antiferromagnetic compositions.

(2) Description of the prior art A significant area of magnetic compositions, initiated with the development of the spinel ferrites during World War II, has expanded over the years to include the related oxidic materials of the orthoferrite and garnet structures. Such materials have been of intense interest for a variety of applications by reason of their combination of high electrical resistivity and magnetic properties which latter may be varied over a broad range of values of magnetization, coercivity, etc. Such magnetic compositions initially of interest as inductor materials and later in microwave circuitry by reason of their square loop and isolation properties, have grown in importance and are now of interest in a vast array of devices. Non-magnetic members of these structures, particularly of the garnet structure, have also been under intense investigation and many such materials are regularly offered as commercial products by a number of suppliers for use as laser hosts, gem stones, etc.

Together, compositions of the included structures are of concern in devices involving electromagnetic radiation from the kilohertz region through the light spectrum into the ultraviolet. For magnetic use they have served such varied devices as inductors and transformers and in such sophisticated devices as bubble domain switching and memory elements. In the latter, dependence is had on regions of cross-sections of a few square mils or less having magnetic polarization opposite to that of surrounding regions.

While many of the concerned crystalline materials are regularly grown by such techniques as Czochralski growth and hydrothermal growth, many are most expediently grown by random nucleation or on a seed from the flux. In fact, from a commercial standpoint, many of the included compositions are regularly grown from the flux and it is expected they will be for many years.

Partly as a result of the intense and varied interest in the included compositions, flux growth techniques have undergone significant development over a period including the last decade. The desire to improve flux growing procedures has had its impact on growing apparatus including furnace design. Other developments have been concerned with the nature of the flux composition itself. Particularly in the garnet field, growth procedures have been advanced by successive improvement of flux compositions starting with simple PbO and proceeding through very closely controlled multicomponent systems containing such additives as PbF B 0 and others sometimes singly, sometimes is combination.

Fundamental and applied study of fiux growing compositions continues with a view to growth of larger, more perfect crystals and also to controlled orientation.

SUMMARY OF THE INVENTION It has been found that the addition of very small, closely controlled amounts of vanadium pentoxide (V 0 to fluxes containing PbO results in a decrease in nucleation sites with a concomitant increase in crystal size. For comparable growth conditions and for fluxes modified only by such addition, this increase in size resultsv for all of the PbO-containing flux systems containing at least 30 percent by weight of PbO. Reduction of nucleation sites is also useful in seeded growth since spurious nucleation is thereby minimized or eliminated. Optimization of the V O -containing fiux systems results in growth of some of the largest, most perfect crystals thus far reported, and such crystal product as well as devices dependent for their operation on such product is included in the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a spontaneously nucleated crystal having natural faces as grown from a flux of the invention; and

FIGS. 2 and 3 are a schematic portion of a magnetic device utilizing a crystalline material grown in accordance with the invention.

DETAILED DESCRIPTION (1) The figures The crystal of FIG. 1 is of dimensions of approximately 1 inch by 1 inch by inch grown by a flux technique of the invention. The particular crystal depicted is an orthoferrite, and one such specimen was, in fact, grown under the processing conditions set forth in Example 1. From a magnetic device standpoint, it is interesting to note that the natural plate depicted is a C plate so that the uniaxial easy direction of magnetization for temperatures above the reorientation temperature lies normal to the major face of the plate. Crystals of this morphology more usually result by use of the V O -containing fluxes herein than by other flux techniques which have been reported. This orientation is particularly useful in devices of the nature of that represented by the succeeding figures.

The device of FIGS. 2 and 3 is illustrative of the class of bubble devices described in IEEE Transactions on Magnetics, vol. MAG-5, No. 3, 544-553 (September 1969) in which switching, memory and logic functions depend upon the nucleation and propagation of enclosed, generally cylindrically shaped, magnetic domains having a polarization opposite to that of the immediately surrounding area. Interest in such devices centers, in large part, on the very high packing density so afforded, and it is expected that commercial devices with from 10 to 10 bit positions per square inch will be commercially available. The device of FIGS. 2 and 3 represent a somewhat advanced stage of development of the bubble devices and include some detail as to printed circuit overlap which has been utilized in recently operated devices.

FIG. 2 shows an arrangement 10 including a sheet or slice 11 of material in which single wall domains ca be moved. The movement of domains, in accordance with this invention, is dictated by patterns of magnetically soft overlay material in response to reorienting in-plane fields. For purposes of description, the overlays are bar and T-shaped segments, and the reorienting in-plane field rotates clockwise in the plane of sheet 11 as viewed in FIGS. 2 and 3. The reorienting field source is represented by a block 12 in FIG. 2 and may comprise mutually orthogonal coil pairs (not shown) driven in quadrature as is well understood. They overlay configuration is not shown in detail in FIG. 1. Rather, only closed information loops are shown in order to permit a simplified explanation of the basic organization, in accordance with this invention, unencumbered by the details of the implementation. We will return to an explanation of the implementation hereinafter.

The figure shows a number of horizontal closed loops separated into right and left banks by a vertical closed loop as viewed. It is helpful to visualize information, i.e., domain patterns, circulating clockwise in each loop as an in-plane field rotates clockwise. This operation is consistent with that disclosed in the aforementioned publication and is explained in more detail hereinafter.

The movement of domain patterns simultaneously in all the registers represented by loops in FIG. 1 is synchronized by the in-plane field. To be specific, attention is directed to a location identified by the numeral 13 for each register in FIG. 1. Each rotation of the in-plane field advances a next consecutive bit (presence or absence of a domain) to that location in each register. Also, the movement of bits in the vertical channel is synchronized with this movement.

In normal operation, the horizontal channels are occupied by domain patterns and the vertical channel is unoccupied. A binary word comprises a domain pattern which occupies simultaneously all the positions 13 in one or both banks, depending on the specific organization, at a given instance. It may be appreciated that a binary word, so represented, is fortunately situated for transfer into the vertical loop.

Transfer of a domain pattern to the vertical loop, of course, is precisely the function carried out initially for either a read or a write operation. The fact that informa tion is always moving in a synchronized fashion permits parallel transfer of a selected word to the vertical channel by a simple expedient of tracking the number of rotations of the in-plane field and accomplishing parallel transfer of the selected word during the proper rotation.

The locus of the transfer function is indicated in FIG. 2 by the broken loop T encompassing the vertical channel. The operation results in the transfer of a domain pattern from (one or) both banks of registers into the vertical channel. A specific example of an information transfer of a one thousand bit word necessitates transfer from both banks. Transfer is under the control of a transfer circuit represented by block 14 in FIG. 2. The transfer circuit may be taken to include a shift register tracking circuit for controlling the transfer of a selected word from memory. The shift register, of course, may be defined in material 11.

Once transferred, information moves in the vertical channel to a read-write position represented by vertical arrow A1 connected to a read-write circuit represented by block 15 in FIG. 2. This movement occurs in response to consecutive rotations of the in-plane field synchronously with the clockwise movement of information in the parallel channels. A read or a write operation is responsive to signals under the control of control circuit 16 of FIG. 2 and is discussed in some detail below.

The termination of either a write or a read operation similarly terminates in the transfer of a pattern of domains to the horizontal channel. Either operation necessitates the recirculation of information in the vertical loop to positions (13) where a transfer operation moves the pattern from the vertical channel back into appropriate horizontal channels as described above. Once again, the information movement is always synchronized by the rotating field so that when transfer is carried out appropriate vacancies are available in the horizontal channels at positions 13 of FIG. 2 to accept information.

For simplicity, the movement of only a single domain, representing a binary one, from a horizontal channel into the vertical channel is illustrated. The operation for all the channels is the same as is the movement of the absence of a domain representing a binary zero. FIG. 3 shows a portion of an overlay pattern defining a representative horizontal channel in which a domain is moved. In particular, the location 13 at which domain transfer occurs is noted.

The overlay pattern can be seen to contain repetitive segments. When the field is aligned with the long dimension of an overlay segment, it induces poles in the end portions of that segment. We will assume that the field is initially in an orientation as indicated by the arrow H in FIG. 3 and that positive poles attract domains. One cycle of the field may be thought of as comprising four phases and can be seen to move a domain consecutively to the positions designated by the encircled numerals 1, 2, 3 and 4 in FIG. 3, those positions being occupied by positive poles consecutively as the rotating field comes into align ment therewith. Of course, domain patterns in the channels correspond to the repeat pattern of the overlay. That is to say, next adjacent bits are spaced one repeat pattern apart. Entire domain patterns representing consecutive binary words, accordingly, move consecutively to positions 13.

The particular starting position of FIG. 3 was chosen to avoid a description of normal domain propagation in response to rotating in-plane fields. That operation is described in detail in the above-mentioned application of Bobeck. Instead, the consecutive positions from the right, as viewed in FIG. 2, for a domain adjacent the vertical channel preparatory to a transfer operation are described. A domain in position 4 of FIG. 3 is ready to begin its transfer cycle.

(2) Compositional considerations From the generic standpoint, it is observed that improvement in crystal size invariably results in the growth of any crystalline material of the included class use of any PhD-containing fiux containing at 'least 30 weight percent of PbO to which at least .0005 weight percent of V 0 has been added. In this and all subsequent descriptive matter, flux ingredients are expressed in terms of weight percent based on total flux. The only general requirement implied by this statement is that the nutrient to flux ratio be such as to, in fact, yield crystalline material either by a dropping temperature method or by change in the nutrient-toflux ratio as by, for example, evaporation. Extensive experimentation has shown at least this advantage to obtain for any growth arrangement utilizing any included flux composition in which the only change is the addition of V 0 Nevertheless, optimization both as to V 0 content, as to relative amounts of other flux ingredients where more than a binary flux is used, and of overall nutrient-to-fiux ratio is desirable for product optimization. While some of these considerations are already part of a prior art and while others are simply determined upon revelation of the invention concept, general and preferred ranges, at least of illustrative systems, are set forth for the assistance of the practitioner.

'(a) V 0 content.It has been stated that the amount of 0.0005 percent by weight represents the minimum usable V 0 content. Unlikely as it may seem, amounts of this small magnitude have a marked effect on the number of nucleation sites and, therefore, result in increased crystal size. Further reduction in V 0 content, however, while it has some slight effect, is not considered to be of significant utility in this respect. A broad maximum of about 3.0 percent by weight is determined by the observation that larger amounts, at least in certain flux systems, result in production of amounts of unwanted vanadium-containing phases in the growing crystal (over a V range of from 0.0005 percent to 3.0 percent, analysis of grown crystals indicates total absence of vanadium to a tolerance of 5 p.p.m.). A preferred V 0 range is considered to lie within the limitation of from 0.0015 Weight percent to about 0.005 weight percent. The preferred minimum results in further significant minimization of nucleation sites as compared with the broad minimum while the preferred maximum is suggested by the fact that little further beneficial effect results by exceeding this amount.

(b) Ternery flux compositions.-While, in general, ratios of flux ingredients excluding V 0 proven useful in the past continue to be useful with the addition of V 0 extensive experimentation has shown some shift in preferred ranges. One of the more significant flux compositions for use in the practice of this invention contains B 0 in addition to PbO. B 0 content continues to lie within the generally practiced range of 2 weight percent to weight percent (always based on total flux) The preferred range where V 0 is present is roughly defined by the limits of from 3 weight percent to 5 weight percent. -In both cases, minima are based on solubility considerations (increased solubility of nutrient material being one of the chief advantages of the use of B 0 while the maxima are based on formation of extraneous borate phases. While borate formation is not a significant problem above the preferred maximum, little further solubility advantage accrues by use of amounts in excess of this quantity.

(c) lbF .In general, for fluxes containing PbF whether ternary (PbO-PbF -V O or quaternary it is generally accepted that amounts of *PbF less than 2 percent have little eifect on growth characteristics. The maximum PbF content for the ternary system is approximately 70 percent less the V 0 while, for the quaternary system, this is still further reduced by the minimum B 0 content of 5 percent i.e., 65 percent less the V 0 content. A preferred maximum for PbF in the ternary system is approximately 60 percent while, for the quaternary system, the preferred maximum is approximately 50 percent less the amount of V 0 Limits are based on solubility for nutrient at the low end and minimization of further improvement in solubility at the high end.

(d) Other flux ingredients.-The foregoing is largely exemplary although virtually all commercially used fluxes are described. Under special conditions, it is sometimes desirable to include additional ingredients. Such additional ingredients, which may be added in amounts no greater than about 5 weight percent based on total flux, sometimes afford further control of nucleation. An example of such an additional ingredient is CaO which has been utilized in garnet growth (see US. Pat. 3,386,799).

(3) Nutrient It is an unfortunate fact that many of the flux systems do not result in congruent growth, i.e., it is necessary to include relative amounts of nutrients that do not correspond with the desired stoichiometry. This consideration is in no way affected by the addition of V 0 A particular example is the iron-containing garnets where it is necessary to include excess iron oxide in the nutrient. Detailed examination of this case is not considered necessary to the description. In general, in such garnet growth, excess iron oxide in the range of about 2 weight percent to about 10 weight percent based on the stoichiometric indicated amount of iron oxide is included. Lesser excesses usually within the range of from 1 weight percent to 5 weight percent are usefully included in the growth of orthoferrites and spinel ferrites. Non-iron-containing compositions of any of the structures herein as, for example, aluminum or gallium garnets, are generally congruently melting, and

non-stoichiometric ratios of nutrient ingredients are not generally required.

Deviation from any of the above indicated ranges is permitted although, in general, such deviation results in disadvantageous growth, for example, in reduction of total crystalline product. So, for example, in many cases, substantially non-stoichiometric ratios of nutrient ingredients may result in the growth of the desired product although the amount of product is reduced.

(4) Product composition It has been indicated that the three structures of primary concern for the purpose of the invention are the garnet (A B O the orthoferrite (AB03), and the spinel ferrite (AB O All compositions capable of forming these structures are advantageously grown from the fluxes of the invention. The range of compositions so defined is extremely varied and is complicated by combination and substitutions. Some compositions of device interest may include as many as four or more ions in any given crystal site (for example, the dodecahedral site in the garnet may be populated by four or more 4] rare earths). It would be unproductive to attempt to outline all such compositions especially since there is no observable relationship between composition and the advantage realized by use of the flux. The simple, unsubstituted materials may, however, be briefly outlined in the following terms.

A may include one or more of the trivalent ions of Y and any of the 4f rare earths,

B may include one or more of the trivalent ions of Fe,

Al, Ga, Mn, Cr, and Co,

A may include one or more of any divalent ions as well as ions of other valence (which then require balancing); notable examples are Mg, Ni, Cu, Zn, Cd, Mn and Co.

In addition to mixtures of such ions and partial substitutions of ions not set forth, certain substitutions even include compensated pairs of ions including ions having a valence state different from those indicated. For example, useful garnet compositions may utilize the Ca-Si or Ca-Ge A-B pairs in lieu of the trivalent-trivalent cation pair normally included, and other compositions even utilize compensated pentavalcnt ions such as vanadium. An example of the latter includes calcium and bismuth in the dodecahedral sites.

Variations in the discussed structures include lithium ferrite, Li Fe O a spinel-like ferrite conventionally included in the spinel class.

(5 Growth It has been indicated that general growth procedures are unchanged by the V 0 addition. Nucleation may be spontaneous or seeded. It may be brought about by dropping temperature or by change in nutrient to flux ratio as by evaporation. Seeded growth may be accomplished by Czochralski, flux, Bridgeman or other. For illustrative purposes, the general procedure followed in many of the spontaneous nucleation experiments reported in this description is set forth.

A mixture of the starting materials is weighed into a cubic centimeter platinum crucible and is sealed with a platinum lid. The crucible is next placed in a horizontal globar furnace with a silicon carbide mufiie and a mullite floor plate. For expediency, the furnace may be preheated to 1300" C. The crucible, together with its contents, is then permitted to attain a temperature of 1300 C. and is maintained at this temperature for a period of eight hours. For charges of the order of 500 grams when using larger crucibles, it has been found helpful to stir the mixture and so assure complete solution. Without stirring, in a charge of this size, it is observed that stratification of the nutrient materials occurs at the top of the melt which is caused by the large differences in densities of nutrient and flux.

Controlled cooling at the rate of 2 per hour from the maximum of 1300 C. is then commenced by a controlled energization of the furnace. This program is continued until the resolution temperature is reached. At this point, the crucible is removed from the furnace and the still liquid portion is poured off. After pouring off the liquid, the crystals still in the crucible are permitted to cool. This is tantamount to an air quench, cooling taking of the order of one hour to reach ambient temperature.

The crucible is then immersed in a vessel containing a dilute solution of nitric acid and water, of the order of 20 percent acid by volume. The acid cleaning procedure is continued until all flux residue has been removed from the crystals. Under ordinary circumstances, acid cleaning at room temperature takes of the order of three hours, although this is variable, being dependent on the amount of residue, the size of the charge and the number of clusters. It is found expeditious to carry out the acid cleaning at temperatures approximating the boiling point of the acid solution to hasten removal of product. Subsequent to this, the acid solution is poured off, the crucible removed from the container, and the crystals washed in three successive rinses of boiling distilled water, following the water washing, the crystals are dried by airdrying at room temperature.

In the following examples, the resultant crystals were chemically analyzed and magnetic measurements were made on the washed product. Results of these measurements were in conformity with observed properties of specimens of these compositions produced by other methods.

(6) Examples EXAMPLE 1 TmFeO (thulium orthoferrite) 100 cc. platinum crucible Grams Tm O 16.60 R 0, 30.00 as as 2 a PbO 160.00

1300 C. for four hours. Contents cooled at 10 C. per hour, air quenched at 890 C. The product is a single crystal of TmFe 1 inch by 1 inch by A inch with an easy direction of magnetization normal to major faces.

EXAMPLE 2 YFeO (yttrium orthoferrite) 100 cc. platinum crucible Grams Y203 6.00 Fe O 11.25 v.0 0.1500 B203 4.00 PbO 80.00

1300 C. for four hours. Contents cooled at 20 C. per hour, air quenched at 995 C. The product is a single crystal of YFeO inch by inch by inch with an easy direction of magnetization normal to major faces.

8 EXAMPLE 3 Sm Tb FeO (samarium-terbiurn orthoferrite) cc. platinum crucible Grams Sm203 Tb O 4.17 F6 0 v 0 0.2000 B 0 4.00 PM) 80.00

1300 C. for four hours. Contents were cooled at 3.0 C. per hour, air quenched at 925 C. The product is a single crystal of Sm Tb FeO of dimensions Mt inch by inch by /2 inch.

1300 C. for four hours. Contents were cooled at 2.0 C. per hour, air quenched at 700 C. The product is a single crystal of Li Fe O of dimensions about /2 inch on the octahedral edge.

EXAMPLE 5 Y3F5012 '(yttrium iIOn garnet) 100 cc. platinum crucible Grams Y O 6.00 F6203 .Y V 0 0.5000 B 0 6.00 PbO 80.00

1300 C. for five hours. Contents were cooled at 2.0 C. per hour, air quenched at 1020 C. The product is a single crystal of Y Fe O of transverse dimension about inch by inch.

What is claimed is:

1. Method of growing single crystals of a material having a structure selected from the group consisting of garnet, orthoferrite and spinel ferrite, which comprises heating together nutrient materials to yield such crystals in a flux comprising at least 30 percent by weight PbO to form a solution characterized in that said flux additionally contains at least 0.0005 percent by weight of V 0 based on the total flux composition.

2. Method of claim 1 in which the V 0 content is within the range of from 0.0005 to about 3.0 percent on the same basis.

3. Method of claim 2 in which the V 0 content is from about 0.0015 percent to about 0.005 percent on the same basis.

4. Method of claim 3 in which the said flux contains at least one additional ingredient.

5. Method of claim 4 in which the said additional ingradient is PbF in amount of at least 30 weight percent less the V 0 content on the same basis.

6. Method of claim 4 in which the said additional ingredient is B 0 in amount of at least 2 percent by weight on the same basis.

7. Method of claim 6 in which the maximum B 0 content is 10 percent by weight on the same basis.

8. Method of claim 7 in which the B 0 content lies within the approximate range of from 3 percent to 5 percent.

9. Method of claim 1 in which the said material consists essentially of a garnet of the approximate stoichiometry A B O in which A consists essentially of a trivalent ion of an atom selected from the group consisting of yttrium and the 4f rare earths and B consists essentially of at least one trivalent ion of an atom selected from the group consisting of Fe, Ga and Al.

10. Method of claim 1 in which the said material consists essentially of an orthoferrite of the approximate stoichiometry ABO in which A is a trivalent ion of at least one atom selected from the group consisting of Y and the 4f rare earths and in which B consists essentially of trivalent iron.

11. Method of claim 1 in which the said material consists essentially of a spinel ferrite of a composition select: 15

ed from the group consisting of Li Fe Q and AB O in which A is at least one divalent ion of an atom selected from the group consisting essentially of Mg, Mn, Zn, Cu, Ni, Cd and Co.

10 References Cited UNITED STATES PATENTS 2,370,443 2/1945 Biefeld 23--51 R 2,957,827 10/ 1960 Nielsen 23305 3,050,407 8/1962 Nielsen 23301 R 3,079,240 2/1963 Remeika 23305 3,203,899 8/1965 Fisher 252301.4 R 3,234,135 2/1966 Ballrnan et a1 23305 3,268,452 8/ 1966 Geller 25262.63 3,346,344 10/1967 Levinstein et al. 23301 SP 3,386,799 6/1968 Grodkiewicz 2351 R 3,523,901 8/ 1970 Amemiya et al 25262.62

NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner US. Cl. X.R.

12. Single crystal prepared in accordance with the meth- 20 254 62.62

0d of claim 1. 

